Atomic Layer Deposition (ALD) is a relatively old process that deposits near-perfect layer-by-layer films onto surfaces based on sequential self-limiting surface reactions. Early ALD processes were demonstrated in the 1970s through 1990s (e.g., U.S. Pat. No. 4,058,430; U.S. Pat. No. 4,389,973; Kumagai, H. et al., Comparative study of Al2O3 optical crystalline thin films grown by vapor combinations of Al(CH3)3/N2O and Al(CH3)3/H2O2, Jpn. Appl. Phys., 32, 6137 (1993); all of which are hereby incorporated by reference in their entireties.
The following is a general description of an ALD process:
Step 1.Gas Molecule A + Reactant gas↓Surface A (Single Layer) + By-product gasStep 2.PurgeStep 3.Gas Molecule B + Reactant gas↓Surface B (Single Layer on A) + By-product gasStep 4.Purge
The deposition process is normally preceded by a light N2 plasma clean. Molecule A is typically a metallo-organic species. Species A and B react with one another. Steps 1-4 are typically repeated a number of times in order to produce an ALD film with composition ABABAB . . . . It should be noted that the A-B sequence may terminate with either A or B as the top layer; additional layers may be formed on the ALD film, some examples of which are described below. Thickness is controlled by the number of AB sequences. Most ALD films are dielectric, but conductive and bilayer films can also be produced.
In one specific example of an ALD process that produces an aluminum oxide (“alumina”) film, Molecule A is Al(CH3)3 while Molecule B is H2O. The reactions are as follows:
where the asterisk (*) indicates a surface species. In this example, each AB layer adds approximately 1.25 angstroms of thickness to the film.
Published data suggests that 25 angstroms of alumina ALD on polymer films (including polyimides) does not significantly reduce gas and water permeation, 50 angstroms provides approximately 10-times reduction in permeation rates, and 100 angstroms reduces moisture transmission by more than 100 times (see Groner, M. D. et al., Gas diffusion barriers on polymers using Al2O3 atomic layer deposition, Appl. Phys. Letters, 88(5) 051907 (2006), which is hereby incorporated herein by reference in its entirety).
In another specific example, an ALD film is formed of Al2O3—TiO2 bilayers (i.e., alternating layers of Al2O3 and TiO2). Experimental data suggests that such alumina-titania bilayer films are better barriers than films formed with just one of these materials. Such ALD films may be deposited at approximately 60° C., although higher temperature processes would generally produce denser films with better electrical properties.
Traditionally, ALD processes were run at high temperatures (e.g., near or above 500° C.). However, there have been a number of papers published showing that ALD films can be produced at lower temperatures (e.g., <200° C.). A University of Colorado group has published extensively using a 177° C. process and have shown process capabilities as low as 35° C. See, for example, Hoivik, Nils D. et al., Atomic layer deposited protective coatings for micro-electromechanical systems, Sensors and Actuators A 103 (2003) 100-108; Hermann, C. F. et al., Hydrophobic Coatings Using Atomic Layer Deposition and Non-Chlorinated Precursors; and Herman, Carl F. et al., Conformal hydrophobic coatings prepared using atomic layer deposition seed layers and non-chlorinated hydrophobic precursors, J. Micromech. Microeng. 15 (2005) 984-992; all of which are hereby incorporated herein by reference in their entireties. Generally speaking, ALD films deposited at low temperature are amorphous, tend to retain some carbon and hydrogen, and typically have lower dielectric strength than high temperature ALD films.